The REST API methods for entity creation and modification have been
simplified with more basic entities.

In addition, the UserResource class has been added so that it is
possible now for clients to use it for user authentication.

In order to avoid repeated pets in the relation.

Some JavaDoc errors detected in the entities have been fixed.

The REST layer has been added but with incomplete tests. Students will
have to complete the tests.

This commit includes an important refactorization regarding tests. Test
classes and datasets have been moved from the "service" project to a new
"tests" project. This way, any project can include this "tests" project
as a dependency in order to have access to these common testing
resources.

The "administrator" entity has been added to represent the
administrators of the application.

This commit adds the test cases for the domain entities.

This commit includes the complete code for the domain entities.